Whisk eggs and sugar, then add salt, yogurt, melted butter, flour and baking powder. Lightly butter an oven cake tin, sprinkle with grepefruit rinds. Then cut 1 of the 2 grepefruits discarding the seeds and the white bits. Fold them with the mixture and pour on the cake tin. Bake at 180 C for about 40 minutes or until golden on top and cooked through. To decorate the cake, slice another grapefruit and gently heat it with the rose water and the vanilla essence for just a couple of minutes. Garnish with mint leaves.